The invention relates to a device for moving a gas at subatmospheric pressure.
A known device for moving a gas at subatmospheric pressure has a rotor in a stator in a housing. The rotor and/or the stator is provided with a screwthread-like structure which moves a gas between the rotor and stator when they relatively rotate.
From published German patent application DOS No. 22 55 618, it is known that devices of this type may have different functions, depending on their design. In one function, they form a stage of a vacuum pump; in another function, they serve as a molecular or viscous seal.
When the device forms a stage of a vacuum pump, it is usually known as a molecular pump. This is classed as a drag pump, the principle of operation of which in relation to a turbomolecular pump is described on pp. 202 et seq. of the textbook Theorie und Praxis der Vakuumtechnik (Theory and Practice of Vacuum Technology), by Wutz, Adam and Walcher. In general, a moving rotor surface and a stationary stator surface are so designed and spaced apart that the impulses transmitted by the surfaces to gas molecules preset between them have a preferential direction. As a rule, this is done by providing the rotor and/or the stator surface(s) with screwthread-like spiral or helical recess or projection structure for obtaining the preferential direction. Molecular pumps operating on this principle are known from German patent Nos. 605,902, 625,444 and 912,007, from Swiss patent Nos. 101,871 and 222,288, and from British patent No. 332,879. Moreover, a combination molecular and turbomolecular pump is known from published German patent application DOS No. 24 12 624.
Published German patent applications DOS Nos. 22 555 618 and 30 01 134 give examples showing the device as a molecular or viscous seal. As such, it serves for tightly sealing a space that is below atmospheric pressure, i.e., under vacuum. In such application, the depth and width of the screwthread-like structure usually are made considerably smaller than in the former application because these quantities affect the gas-moving efficiency which, in the case of the sealing function, can be low.
It is apparent from the art described that such devices have been proposed in many different varieties and designs for molecular pumps and seals. Nevertheless, they have failed, so far, to gain market acceptance. The main reason for this lies in technological difficulties connected with their construction. Above all, the clearance between the relatively-moving rotor and stator surfaces must be extremely small to limit backstreaming, and the temperature sensitivity of such devices is a definite drawback in this regard. With pronounced temperature variations, the relatively-moving rotor and stator surfaces may come into contact on account of their small clearance. Moreover, even minor temperature variations thermally change the rotor and/or stator dimensions enough to change markedly their small clearance. Both result in significantly altered pumping or sealing characteristics.
The changes in the operating conditions which thus occur as a result of even minor temperature variations have an adverse effect on the operating behavior. With turbomolecular vacuum pumps, such difficulties are less pronounced, which is why they have been most successful in maintaining their market position for the last ten years or so, but these factors explain why the advantages offered by molecular pumps and seals have not been exploited commercially up to now. These advantages are, in the main, that a molecular pump or seal can be used over a wide range of pressures. Molecular vacuum pumps, for example, have good pumping characteristics, not only in the molecular flow region (high and ultrahigh vacuum), but also and especially in the viscous flow region (medium-high and low vacuum).